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    Cache access time

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    A single level cache has following specifications.

    Access time = 2.5 ns
    Line size = 64 bytes
    Hit ratio = 0.95

    Main memory uses a block transfer capability, and has first word (4 bytes) access time of 50 ns and access time for following words as 5 ns.

    Compute the access time when there is a cache miss? Assume that the cache waits until the line has been fetched from main memory and then re-executes for a hit.

    Will increasing the line size to 128 bytes, and a resulting increase in H (hit ratio) to .97, reduce the average memory access time?

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    Solution Preview

    Block size = Line size = 64 bytes = 64/4 or 16 (4 bytes) words

    Time to fetch a line (or transfer a block) from main memory
    = 50 ns to transfer first word + (15 * 5) ns to transfer remaining 15 words
    = 50 + 75
    = 125 ns

    Cache access time on a cache miss
    = time to completely fetch a line from ...

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